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1.6.2Physical Chemistry

Balanced Equations — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 26 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Part 1 of 6What a Balanced Equation Shows

1.6.2 Balanced Equations
Conservation of mass
  • The total number of atoms of each element is the same on both sides because atoms are neither created nor destroyed.
Coefficients
  • The big numbers in front of formulae tell you the ratio in which moles of substances react.
Subscripts
  • The small numbers inside formulae show how many atoms are in one formula unit and must never be changed to balance.
Charge balance
  • In ionic equations the total charge on each side must also be equal.

Part 2 of 6Balancing Step by Step

ExampleC₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l) is balanced with 3 C, 8 H and 10 O on each side.

1
Write the correct formulae for all reactants and products first.
2
Count the atoms of each element on both sides.
3
Balance elements that appear in only one substance on each side, leaving hydrogen and oxygen until last.
4
Adjust coefficients one at a time, recounting after each change, and use fractions only as a temporary step.
5
Finish by checking every element and adding state symbols.

Part 3 of 6State Symbols and Ionic Equations

State Symbols and Ionic Equations
State symbols
  • (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water.
Spectator ions
  • Ions that are present in the mixture but do not change are left out of the ionic equation.
Example
  • Ag⁺(aq) + Cl⁻(aq) → AgCl(s) shows only the precipitation of silver chloride.
Checking
  • Make sure both mass and charge are balanced in the final ionic equation.
Full equationIonic equation
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)H⁺(aq) + OH⁻(aq) → H₂O(l)

Part 4 of 6Common Reaction Types

Combustion
  • Hydrocarbons react with oxygen to form carbon dioxide and water; incomplete combustion can form CO or C.
Acid and carbonate
  • Gives a salt, water and carbon dioxide, for example CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g).
Acid and metal
  • Gives a salt and hydrogen, such as Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g).
Thermal decomposition
  • One compound breaks down on heating, for example CaCO₃(s) → CaO(s) + CO₂(g).

Part 5 of 6Worked Example

Question Balance the equation: Fe₂O₃(s) + CO(g) → Fe(s) + CO₂(g).

  1. Irontwo atoms on the left, so write 2Fe on the right.
  2. OxygenFe₂O₃ provides three O atoms, and each CO provides one, so use 3CO to give three C atoms.
  3. Three CO produce three CO₂, which keeps carbon and oxygen balanced.
  4. RecountFe 2 and 2, C 3 and 3, O 3 + 3 = 6 and 6.

Answer Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)

Part 6 of 6Exam Checklist & Tips

1.6.2 – Key Points for the Exam

  • Balanced equation An equation with equal numbers of each type of atom on both sides.
  • Coefficient The number in front of a formula showing the amount of that substance.
  • Spectator ion An ion that does not take part in the reaction.
  • State symbol Letters (s), (l), (g) or (aq) showing the physical state.

Common Mistakes

  • Changing the subscripts in a formula to balance an equation.
  • Leaving out state symbols when the question asks for them.
  • Forgetting to balance charge in an ionic equation.
  • Writing H₂O(aq) for water; pure liquid water is (l).

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